Script '''bracelevel.tcl''':

# prefix the current brace level on each line -[jcw]

proc bracelevel {str} {
  set lev 0
  set out ""
  foreach l [split $str \n] {
    append out $lev \t
    foreach c [split $l ""] {
      append out $c
      switch -- $c \{ { incr lev } \} { incr lev -1 }
    }
    append out \n
  }
  return $out
}

puts [bracelevel [read stdin]]
----
Output of '''tclsh bracelevel.tcl <braclevel.tcl''':

0       # prefix the current brace level on each line -[jcw]
0
0       proc bracelevel {str} {
1         set lev 0
1         set out ""
1         foreach l [split $str \n] {
2           append out $lev \t
2           foreach c [split $l ""] {
3             append out $c
3             switch -- $c \{ { incr lev } \} { incr lev -1 }
3           }
2           append out \n
2         }
1         return $out
1       }
0
0       puts [bracelevel [read stdin]]
0